Classify the three lever systems in the body and calculate mechanical advantage to explain force and range of movement

What this dot point is asking

SEAB wants you to classify the three lever systems found in the body and to calculate mechanical advantage to explain whether a lever favours force or range of movement. The central idea is that bones, joints and muscles form levers, and the way the parts are arranged decides whether the body produces large force or large, fast movement.

The answer

The three parts of a lever

Every lever has three components:

• the fulcrum, the fixed pivot point (a joint in the body);
• the effort, the force applied to move the lever (a muscle pulling);
• the load (resistance), the weight being moved (the body part plus any external weight).

The three classes of lever

Levers are classified by which component sits in the middle.

• First class lever: the fulcrum is in the middle (effort-fulcrum-load). Example: nodding the head, where the neck joint is the fulcrum between the neck muscles (effort) and the weight of the head (load).
• Second class lever: the load is in the middle (fulcrum-load-effort). Example: standing on tiptoes, where the ball of the foot is the fulcrum, body weight is the load and the calf muscle is the effort. Second class levers favour force.
• Third class lever: the effort is in the middle (fulcrum-effort-load). Example: a biceps curl, where the elbow is the fulcrum, the biceps is the effort and the weight in the hand is the load. Most body levers are third class, and they favour range and speed of movement.

A memory aid is "1-2-3, F-L-E in the middle": for the first, second and third class, it is the Fulcrum, then Load, then Effort that sits in the middle.

Mechanical advantage

Mechanical advantage (MA) measures whether a lever magnifies force or movement. It is the ratio of the effort arm (fulcrum to effort) to the load arm (fulcrum to load):

• MA greater than 1: the lever favours force; it moves a large load with a smaller effort, but with less range and speed. Second class levers behave this way.
• MA less than 1: the lever favours range and speed; it produces a large, fast movement at the load, but the muscle must supply a large effort. Most third class levers behave this way.

Examples in context

Example 1. A footballer rising onto tiptoes to head a ball. The ankle acts as a second class lever: the ball of the foot is the fulcrum, body weight the load and the calf muscle the effort, with the load between the other two. Because second class levers favour force, the calf can lift the whole body weight efficiently.

Example 2. A javelin thrower extending the arm. The elbow acts as a third class lever, the biceps and triceps providing the effort between the elbow (fulcrum) and the hand (load). Its low mechanical advantage favours range and speed, which is exactly what is needed to whip the javelin away at high velocity.

Try this

• Cue. Identify the lever class for a biceps curl and name the fulcrum, effort and load. (Third class; fulcrum is the elbow, effort is the biceps, load is the weight in the hand.)

• Cue. Calculate the mechanical advantage of a lever with a 0.30 m effort arm and a 0.15 m load arm and say what it favours. ($\text{MA} = 0.30 / 0.15 = 2$, greater than 1, so it favours force.)

• Cue. Explain why third class levers, common in the body, are useful in sport despite needing large muscle effort. (Their low mechanical advantage gives large, fast movement at the end of the limb, ideal for throwing, kicking and striking at speed.)